In a variety of clinical situations it is important to measure certain chemical characteristics of the patient's blood such as pH, concentrations of calcium, potassium ions and hematocrit, the partial pressure of O.sub.2 and CO.sub.2 and the like. (See, for example, Fundamentals of Clinical Chemistry, Tietz, Editor, page 135 et seq., Electrochemistry; page 849 et seq., Blood Gases and Electrolytes; 1976, Saunders Company, Phila.; see also the patent to Battaglia et al. U.S. Pat. No. 4,214,968.) These situations range from a routine visit of a patient in physician's office to monitoring during open-heart surgery. The required speed, accuracy, and similar performance characteristics vary with each situation.
Measurement of chemical characteristics of blood during open-heart surgery provides the most demanding set of criteria. Presently, blood gas analysis during major surgery is provided by repeated transfer of discrete blood samples to a permanent lab-based blood gas analyzer or by use of sensors placed in-line with the extra-corporeal blood circuit of a heart-lung machine employed to bypass the patient's heart.
The transfer of discrete blood samples, required by blood-gas analyzers, inherently increases the risk of contaminating the blood sample with ambient air, which may alter certain of the monitored characteristics. Additionally, since such analyzers are complex and costly devices, they are typically located only in the hospital lab where they need to be operated by a skilled technician, resulting in undesirable delay during surgery, critical care or intensive care. Further, such analyzers employ bubble tonometers to generate a suitable gas reference mixture by dissolving quantities of gases, stored in pressurized free-standing tanks, into the electrolyte solution. While replacement of these gas tanks is infrequently required, it is a cumbersome procedure. Finally, these existing analyzers require cleaning to decontaminate all exposed portions from the prior patient's blood prior to subsequent use.
Heretofore measurements of the concentration of dissolved carbon dioxide in body fluids such as blood samples has been carried out using a Stow-Severinghous CO.sub.2 electrode. This device operates on the principle that CO.sub.2, when dissolved in an aqueous solution containing bicarbonate ions, will alter the pH of that solution in a manner which is proportional to the concentration of the dissolved CO.sub.2. A thin film of bicarbonate solution is contained between the chemically sensitive surface of a combination pH electrode, consisting of a pH sensor and a reference sensor, and a gas permeable membrane. When the outer surface of the membrane is surrounded by an aqueous fluid containing dissolved carbon dioxide, the concentration of CO.sub.2 dissolved in the thin film of solution will rapidly equilibrate with the CO.sub.2 concentration outside of the membrane. The pH of the bicarbonate solution, which is now proportional to the CO.sub.2 concentration at the outer surface of the membrane, is measured using the pH sensor internal to the membrane, and thus the device has an electrical output which is proportional to CO.sub.2 concentration.
The structure of the Stow-Severinghous CO.sub.2 electrode can therefore be summarized as a gas permeable membrane under which is contained a layer of aqueous electrolyte solution containing bicarbonate ions, combined with internal pH and reference electrodes which together measure the pH of the bicarbonate solution, and thus the CO.sub.2 concentration. While this device is functionally effective, it is a relatively complex structure. In particular the Stow-Severinghous CO.sub.2 electrode is not well adapted to automated manufacture. With the trend in medical appliances and supplies toward disposables, automated manufacture with the accompanying reduction in production costs is believed to be highly advantageous.
A further problem exists with some types of CO.sub.2 sensors. In many of these sensors the aqueous electrolyte solution containing bicarbonate ions requires the sensor to be hydrated during storage prior to use. In particular, some of these sensors use a gel to provide the bicarbonate ions. This gel must be kept hydrated in storage prior to use in order to remain useful. It would be advantageous to provide a sensor which can be stored in an anhydrous state to be rehydrated prior to the first use. Such a feature would be particularly advantageous for disposable appliances.
Accordingly there is a need for an inexpensive, easily manufactured electrode assembly for the measurement of dissolved CO.sub.2 in body fluids such as blood samples which is storable in an anhydrous state and is adaptable for disposable use. Such a CO.sub.2 electrode could be employed in a set of sensor electrodes for the measurement of a variety of chemical characteristics of the sample.